The present invention relates generally to multilayer optical coatings for transparent substrates, and more particularly to such coatings wherein the layers are deposited by DC reactive sputtering.
DC reactive sputtering is the process most often used for large area commercial coating applications. Metal oxide layers, for example, are deposited by sputtering the appropriate metal in an atmosphere including oxygen. In the reactive sputtering process, the articles to be coated are passed through a series of in-line vacuum chambers each including sputtering sources. The chambers are isolated from one another by vacuum locks. Such system may be referred to as an in-line system or simply a glass coater.
Multilayer antireflection coatings are made by depositing two or more layers of transparent dielectric materials on a substrate. At least one of the layers has a refractive index higher than that of the substrate on which the coating is deposited. The coatings may be designed to reduce reflection at all wavelengths in the visible spectrum. The coatings may yield reflection values less than 0.25 percent over the visible spectrum.
The time it takes to deposit a coating is determined mainly by the number of layers and the sputtering rate of the materials. The use of a glass coater to deposit multilayer antireflection coatings can significantly reduce their cost and thus extend their range of application. Such coatings may be used on picture frame glass and a display case, and as thermal control coatings for architectural and automobile glazings.
Most multilayer antireflection coatings are derived from a basic three layer system. The first or outermost layer of the system has a refractive index lower than that of the substrate and an optical thickness of about one-quarter wavelength at a wavelength of about 520 nanometers (nm). The second or middle layer has a refractive index higher than that of the substrate, and an optical thickness of about one-half wavelength at a wavelength of about 520 nm. The third layer, i.e. the layer deposited on the substrate, has a refractive index greater than that of the substrate but less than the refractive index of the second layer. The optical thickness of the third layer is about one-quarter wavelength at a wavelength of about 520 nm. This basic design was first described in the paper entitled "Three Layered Reflection Reducing Coatings", J. Opt. Soc. Am., Vol. 37, Lockhart and King, pp. 689-694 (1947).
A disadvantage of the basic three layer system is that the refractive indices of the layers must have specific values in order to produce the lowest reflectivity. The selection and control of the refractive index of the third layer is particularly important. Departure from specific values of the refractive index can not be compensated for by varying the thicknesses of the layers.
A simple improvement on the Lockhart and King system is described in U.S. Pat. No. 3,432,225, issued to Rock. The Rock system includes four layers. The first or outermost layer has a refractive index lower than that of the substrate and an optical thickness of about one-quarter wavelength at a wavelength of about 520 nm. The second or middle layer has a refractive index higher than that of the substrate and an optical thickness of about one-half to six-tenths of a wavelength at a wavelength of about 520 nm. The third layer has a thickness of about one-tenth of a wavelength at a wavelength of about 520 nm. The refractive index of the third layer is less than that of the second layer. The fourth layer has an optical thickness of about one-tenth of a wavelength at a wavelength of about 520 nm, and a refractive index greater than that of the third layer and the substrate. The third layer may be the same material as the first layer and the fourth layer may be the same material as the second layer.
The Rock system may be used with different combinations of materials. Differences in refractive indices may be compensated for by different layer thicknesses. Magnesium fluoride (MgFl) can be used to form the outer and third layers. If a higher refractive index material were used for the outer layer, then the refractive index of the second layer would also need to be higher to produce the lowest reflectivity.
Magnesium fluoride may be deposited by sputtering but requires a reactive atmosphere including fluorine or hydrogen fluoride. In a layer system, designed for deposition by DC reactive sputtering in a glass coater, the outer layer is usually silicon dioxide (SiO.sub.2). Silicon dioxide has a refractive index of about 1.46 at a wavelength of about 520 nm. If the reactive index of the first layer is about 1.46, a second layer having a refractive index of about 2.35 would provide the lowest reflection over the visible spectrum. Titanium dioxide (TiO.sub.2) has a refractive index of about 2.35 at a wavelength of about 520 nm. As such, it is commonly used as the high refractive index material in a system deposited by DC reactive sputtering.
The Rock system may require approximately equal thicknesses of titanium dioxide and silicon dioxide. Silicon dioxide may be sputtered four times faster than titanium dioxide. In order to operate at optimum speed, a glass coater may require four times as many sputtering cathodes for titanium dioxide as for silicon dioxide. However, the coater may not have enough chambers to accommodate all of these titanium dioxide cathodes. Thus, the deposition rate for the silicon dioxide will have to be reduced to "keep pace" with the deposition rate of the titanium dioxide. This reduces output and increases production costs.
Materials such as tin oxide (SnO.sub.2) and zinc oxide (ZnO) may be deposited by DC reactive sputtering at a rate at least six times faster than that of titanium oxide. These materials, however, have a refractive index of about 1.9 at a wavelength of 520 nm. The photopic reflection of a four layer antireflection coating using zinc oxide or tin oxide as the second layer would only be about 0.4 percent. The low DC reactive sputter rate for titanium dioxide also presents difficulties in the deposition of highly-reflecting coatings. Highly-reflecting coatings include enhanced metal reflectors which have a metal layer overcoated with low and high refractive index materials. Enhanced reflectors may have four such overcoating layers, including two high and two low refractive index layers arranged alternately. The overcoating layers each have an optical thickness of about one-quarter wavelength at a wavelength of about 520 nm. A low refractive index layer is in contact with the metal layer. The refractive index of the high refractive index material should be as high as possible and the refractive index of the low refractive index material should be as low as possible. This provides optimum reflection enhancement. The high and low index materials should also not absorb visible light.
In the architectural glass coating industry, zinc oxide is used as a layer material for the formation of low emissivity (E) coatings. These coatings are designed to reflect long wavelength infrared radiation while transmitting visible light. They also have a low reflection for visible light from at least one surface. A low E coating may comprise three layers, for example, a silver layer bounded by two dielectric layers. The silver layer must be thick enough to provide high infrared reflection. The dielectric layers reduce reflection from the silver layer in the visible spectrum and thus enhance light transmission through the coated glass. The silver layer is preferably between about 10 and 15 nm thick. The refractive index of the dielectric layers should be relatively high. A material with a refractive index of about 2.35 at a wavelength of about 520 nm, such as titanium dioxide, would be preferred over zinc oxide for the reflection-reducing layers.
Zinc oxide has a refractive index of about 1.9 at a wavelength of about 520 nm. The higher index permits the use of a thicker silver layer while maintaining low visible light transmission and high reflection. For example, using zinc oxide dielectric layers, a silver layer about 8 nm thick produces the lowest reflectivity. For titanium oxide dielectric layers, a silver layer about 130 nm thick produces the lowest reflectivity. The thicker silver layer may also provide higher reflection at longer wavelengths, i.e. a lower emissivity. The thicker silver layer may also provide a higher reflection and lower transmission in the near infrared spectrum, reducing the solar heat load. The industry preference for zinc oxide is substantially based on its high sputtering rate. The lower production costs afforded by the high sputtering rate may be sufficient to justify the less than optimum optical performance. The durability of a layer system incorporating zinc oxide is also poor due to zinc oxide's softness.
It is widely believed that materials which can be deposited at high rates by DC reactive sputtering have relatively low refractive indices. Deposition rate comparisons may be slightly inconsistent from source to source. The type of machine and cathode used may also influence the results. The following approximate rate comparisons serve to illustrate the generalization. The refractive index values cited are the approximate values at a wavelength of about 520 nm. Titanium dioxide has a refractive index of about 2.35, and tantalum oxide (Ta.sub.2 O.sub.5) has a refractive index of about 2.25. Tantalum oxide may be deposited at about twice the rate of titanium dioxide. Zirconium oxide (ZrO.sub.2) has a refractive index of about 2.15 and may be deposited at about twice the rate of titanium dioxide. Tin oxide has a refractive index of about 1.95 and may be deposited at about ten times the rate of titanium dioxide. Zinc oxide has a refractive index of about 1.90 and may be deposited at about ten times the rate of titanium dioxide.
The above-described relationship between the refractive index and the sputtering rate implies that high deposition rates are achievable only with materials having relatively low refractive indices. A more probable explanation, however, is that zinc oxide and tin oxide are semiconductors. As such, oxide build-up on a sputtering target of zinc or tin may not create an insulating layer which lowers the sputtering rate. High oxygen-flow rates are thus possible in the sputtering gas without loss of power. As such, higher oxide deposition rates are possible.
Accordingly, it is the object of the present invention to provide a high refractive index material having a refractive index which is approximately equal to that of titanium dioxide but which may be deposited by DC reactive sputtering at a rate at least four times faster than titanium dioxide and preferably at a rate comparable to zinc oxide.
It is a further object of the present invention to provide a high refractive index material having a high DC reactive sputtering rate which is durable and abrasion resistant.
It is another object of the present invention to provide a simple antireflection layer system which may be deposited at low cost in a large area, in-line sputtering machine.
It is yet another object of the present invention to provide a simple enhanced reflector coating which may be deposited at low cost in a large area in-line sputtering machine.